(a) Field of the Invention
The invention relates to adaptive noise filtering systems, and more particularly, concerns a system adapted to filter a sinusoidal and/or correlated interference signal mixed with a desired signal in an incoming signal. The instant system is especially effective when the interference signal to be removed is contained in the same frequency spectrum as the desired signal.
(b) Description of the Prior Art
Heretofore, various devices and systems have been proposed for the purpose of separating, enhancing or cancelling noise, intersymbol interference, echo or different signal frequencies from an incoming input signal. Those previous systems are exemplified by U.S. Pat. No. 4,052,559 of Paul et al and U.S. Pat. No. 4,238,746 of McCool et al which each teach the use of an adaptive transversal filter incorporating the weight vector which is governed by the Widrow-Hoff algorithm, the latter being a real time least mean square (LMS) approximation of the Weiner-Hoff filter. Such algorithm is described in "Adaptive Noise Cancelling: Principles and Applications" of Widrow et al, in proceedings IEEE, Volume 63, number 12, December 1975, pages 1692 to 1716. However, the Widrow-Hoff algorithm generally requires true linear multipliers which are expensive, on the one hand, and present a system degradation since there are non-ideal on the other hand. Elsewhere, in the intersymbol interference literature many algorithms, such as clipped data or clipped error have been proposed to partly avoid or to reduce the number of those linear multipliers.
Moreover, when the interference signal is merely a sinusoidal waveform, Widrow suggested to use rather a pair of in-phase/in-quadrature signals instead of adaptive transversal filters to track the amplitude and the phase of the sinusoidal interference signals while assuming that the frequency thereof is already known. Even in those cases, several linear multipliers are required to implement the adaptive algorithms.